Since the organic EL devices are self-luminescent devices, they are bright, excellent in visibility, and capable of giving clear display as compared to liquid crystal devices, and studies thereon have actively been conducted.
In 1987, C. W. Tang et al. of Eastman Kodak Co. have turned the organic EL device using an organic material into practical utilization by developing a multilayer structure device wherein various functions are respectively distributed to materials. They laminated a fluorescent material capable of transporting electrons and an organic substance capable of transporting holes, and injected both of the charges into the fluorescent material layer to emit a light, thereby achieving a high luminance of 1,000 cd/m2 or more at a voltage of 10 V or less (see patent document 1 and patent document 2, for example).
Patent Document 1: JP-A-8-48656
Patent Document 2: Japanese Patent No. 3,194,657
Many improvements have hitherto been made for practical application of the organic EL devices, and high efficiency and durability are achieved by an electroluminescence device in which various roles are further subdivided and in which an anode, a hole injection layer, a hole transport layer, an emitting layer, an electron transport layer, an electron injection layer and a cathode are provided in this order on a substrate (for example, see non-patent document 1).
Non-Patent Document 1: Preprints for 9th Workshop of Japan Applied Physics, pages 55 to 61, (2001)
In addition, for the purpose of further improving luminous efficiency, there has been an attempt of utilizing a triplet exciton, and the utilization of phosphorescence-emitting substance has been studied (for example, see non-patent document 2).
Non-Patent Document 2: Preprints for 9th Workshop of Japan Applied Physics, pages 23 to 31, (2001)
The emitting layer may be prepared by doping a charge transporting compound generally called a host material, with a fluorescent substance or a phosphorescence-emitting substance. As described in the above-mentioned workshop preprints, selection of an organic material in the organic EL device has a great influence on various characteristics such as efficiency and durability of the device.
In organic EL devices, charges injected from both electrodes recombine in the emitting layer to thereby provide light emission. However, the mobility of holes is higher than the electron mobility, so that a reduction in efficiency caused by that holes partially pass through the emitting layer becomes a problem. For this reason, an electron transport material having a high electron mobility has been demanded.
Tris(8-hydroxyquinoline) aluminum (hereinafter referred to as Alq3 for brevity), a typical light-emitting material, is generally used as an electron transport material, but the electron mobility thereof is considered to be low. For this reason, 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (hereinafter referred to as PBD for brevity) and the like have been proposed as materials having a high mobility (for example, see non-patent document 3).
Non-Patent Document 3: Jpn. J. Appl. Phys., 27, L269 (1988)
However, it is pointed out that PBD is poor in stability in a thin film state, such as easy occurrence of crystallization, and various oxadiazole derivatives have been proposed (for example, see patent documents 3 to 5).
Patent Document 3: Japanese Patent No. 2,721,442
Patent Document 4: Japanese Patent No. 3,316,236
Patent Document 5: Japanese Patent No. 3,486,994
These electron transport materials have been improved in stability compared with PBD, but the improvement is not considered yet to be sufficient. From the viewpoint of the balance with the hole mobility, the electron mobility is still insufficient. For these reasons, Alq3 having good stability has been used as the electron transport material in many cases. However, satisfactory device characteristics have not been obtained.
Further, as a measure for preventing holes from partially passing through the emitting layer to thereby improve the probability of charge recombination in the emitting layer, there is a method of inserting a hole blocking layer. As hole blocking materials, there have hitherto been proposed a triazole derivative (for example, see patent document 6), bathocuproine (hereinafter referred to as BCP for brevity), a mixed-ligand complex of aluminum (BAlq) (for example, see non-patent document 2) and the like.
Patent Document 6: Japanese Patent No. 2,734,341
However, these materials are each lacking in film stability, or insufficient in the function of blocking holes. The hole blocking material which has been generally used at present is BCP. However, this compound is not considered to be a sufficiently stable material, so that it is not considered to sufficiently function as the hole blocking layer. Thus, satisfactory device characteristics have not been obtained.
In order to improve the device characteristics of organic EL devices, an organic compound excellent in electron injection/transport performance and in hole blocking property and high in stability in a thin film state has been demanded.